In recent years, state and federal regulations have continued to decrease the limits of certain nutrients, particularly phosphorus and nitrogen, that can be discharged from wastewater treatment facilities into streams, lakes, etc. A great deal of emphasis has been placed on reducing the nitrogen discharge levels to below 3 ppm.
In order to efficiently remove nitrogen, many wastewater treatment facilities have been designed to remove nutrients, nitrogen and phosphorus, biologically. For example, one is referred to the disclosures of U.S. Pat. Nos. 4,056,465 and 3,964,998 that disclose two types of biological wastewater treatment processes.
All the biological nitrogen removing systems are based on a combination of nitrification and denitrification processes. During nitrification, ammonia is oxidized to nitrate and nitrites. During denitrification the nitrates and the nitrites are converted to the molecular nitrogen which is then released to the atmosphere. The denitrifying bacteria are capable of using either molecular oxygen or nitrate oxygen as a terminal electron acceptor when they oxidize organic compounds. Under anoxic conditions (such as in the absence of free molecular oxygen), denitrifying bacteria reduce nitrate by a process called nitrate dissimilation in which nitrate or nitrite replaces oxygen in cell respiration. If both oxygen as well as nitrate oxygen are present, the denitrification bacteria will first use up the dissolved oxygen for oxidation of organic compounds before using nitrate oxygen.
Nitrogen removal typically involves a system having an anoxic treatment zone followed by an oxic zone with an internal recycle of nitrified mixed liquor from the oxic treatment zone back to the preceding anoxic treatment zone. The problem comes into play because the nitrified mixed liquor recycled from the oxic zone to the preceding anoxic zone contains a significant dissolved oxygen concentration, typically in the range of 2-4 ppm. The dissolved oxygen recycled back to the anoxic zone oxidizes significant portion of the incoming soluble BOD and thus reduces the BOD available for denitrification. This has a harmful and detrimental effect on the denitrification process carried out in the anoxic zone.
Therefore, there is a need for a nitrogen removal process that efficiently utilizes all or substantially all of the incoming soluble BOD to effectuate denitrification by reducing or eliminating the detrimental effect of dissolved oxygen, recycled to the anoxic zone, on the total denitrification process.